This invention relates to improvements in apparatus for stacking sheets, e.g., sheets of paper, plastic or cardboard and/or the like, by means of helical connectors such as coil springs. Typical examples of commodities which can be turned out by resorting to the apparatus of the present invention are pads of overlapping cardboard and/or paper sheets which can be held together by a length of coil spring in such a way that the individual sheets or groups of sheets can be pivoted relative to the other sheets or groups of sheets along one marginal portion of the pad, namely along a marginal portion provided with a row of registering holes or perforations for discrete convolutions of the length of coil spring.
Conventional apparatus of the above outlined character are provided with a guide which is intended to control the position of the coil spring during threading of its convolutions into successive sets of overlapping holes in one marginal portion of the stack or pile of sheets which are to be pivotally connected to each other. The coil spring is moved lengthwise of the one marginal portion and is turned about its axis so that successive convolutions enter successive sets of overlapping holes; the ends of the fully introduced coil spring are thereupon deformed and/or otherwise enlarged at the two ends of the row of sets of overlapping holes.
The just described conventional apparatus normally form part of a semiautomatic or automatic production line which is designed to turn out pads and similar commodities wherein the sheets are held together by coil springs made of metal or plastic material. The production line can further comprise suitable means for converting large panels or webs of paper, plastic and/or cardboard into sheets of desired size and shape, for assembling the thus obtained sheets into stacks of desired thickness (i.e., into stacks each of which contains a predetermined number of identical or different sheets), and for providing one marginal portion (namely the so-called spine) of each stack with a row of perforations. The thus obtained and treated stacks are ready to receive coil springs (hereinafter called spirals for short) which are designed to hold the sheets together but to permit the sheets to pivot relative to one another along the spine of the respective pad. The aforedescribed production line can further comprise means for providing certain sheets (such as the cover and/or the back sheet) with printed matter and/or other information.
The spiral of each of a series of successive pads can form part of a continuous coil spring which is severed as soon as a requisite length thereof has been threaded through the stacks of holes in a pad, and the end portions of the severed part of the continuous coil spring are thereupon bent and/or otherwise deformed or enlarged so that the spiral remains confined in the thus finished pad. The perforations or holes can be provided in the panels or in the web prior to their subdivision into discrete sheets, or subsequent to assembly of requisite numbers of sheets into stacks. Accurate alignment of perforations in each sheet of a pad with the perforations of the neighboring sheets is highly desirable in order to facilitate and simplify predictable threading of convolutions of a continuous coil spring into the rows of perforations in the sheets of each stack.
The threading of a continuous cylindrical coil spring into the rows of holes or perforations in successive stacks involves a turning and simultaneous lengthwise advancement of the coil spring. The rate of lengthwise movement of the continuous coil spring is related to the lead or pitch of its convolutions. Accurate guidance of the advancing and rotating continuous coil spring is important because this contributes to the quality of the pads as well as to the frequency at which the production line turns out acceptable pads. The number of rejects is directly related to the accuracy at which the continuous coil spring is being advanced during threading of its convolutions into the rows of overlapping perforations in successive stacks of sheets.
It is further important to properly select the lead or pitch of the convolutions which form the continuous coil spring. The leader of the advancing coil spring is likely to bend away from the prescribed path if the lead or pitch of its convolutions is too small. Alternatively, an intermediate portion of the advancing continuous coil spring exhibits a tendency to buckle or bend if the lead or pitch of the convolutions is excessive. The purpose of the aforementioned guide means is to prevent the aforedescribed and/or other stray movements of the advancing continuous coil spring, a task which cannot be carried out in a fully satisfactory manner with presently known guide means, especially if the continuous coil spring is to advance at an elevated speed as required in production lines which are intended to turn out huge quantities of finished commodities per unit of time.